(1) Field of the Invention
The present invention pertains to a drag block that is attached over the exterior of, or screwed onto as an integral part of, oil or gas well production tubing which is hung or being run in a cased hole to increase the fluid resistance of the tubing if it is dropped in the hole and descends through fluid contained in the casing. In particular, the drag block is attached to the production tubing to avoid damage to the casing pipe and the tubing when a length of the production tubing having the drag block attached is inadvertently dropped into and through the casing. In addition, the invention will lessen the cost of "fishing" the dropped tubing out of the hole by reducing the chances of its becoming stuck when it hits bottom. By preventing damage to the casing and production tubing the drag block will minimize the cost of retrieving the dropped tubing and will prevent the possible loss of the well.
(2) Description of the Related Art
It is well known in the prior art that gas or oil wells being prepared for production are provided with a casing pipe to prevent the walls of the well from caving in, to seal off any levels of strata containing water that communicate with the drilled well or fresh water zones and to confine any gas or oil to the strata level where they were encountered. Well casings are typically large diameter pipes and are available in a variety of standard sizes, weights, wall thicknesses, and lengths. Individual lengths or joints of casing pipe are connected together end to end as they are inserted downward into the drilled wells by couplings, also known in the art.
Several lengths or joints of casing connected end to end are typically employed in casing an oil or gas well. A joint is typically about 40 feet in length. The total length of joints of casing inserted downward into a drilled hole can reach thousands of feet, and casing will be run to the bottom of the hole and cemented in place. Production tubing is then inserted downward through the casing pipe in the same way that casing is run into the open hole. A pump may then be run down the tubing on rods, which are connected joint by joint at the surface. The uppermost rod, or "polished rod" is then tied on to the "horses head" of the surface pumping unit. Operation of the pumping unit activates the pump and conveys the fluid of the well upward through the production tubing and into the tanks or storage facilities on the surface.
The casing pipe in a gas or oil well is often the most expensive portion of the well's construction, representing as much as one-third of the cost of the oil or gas well. Damage to the casing pipe of a producing well can represent a substantial expense, not only for the repair of the casing pipe but also for the downtime of the productive well. Moreover, damaged casing can make it impossible to run production tubing or other equipment into the well resulting in the loss of the well.
Casing pipe of production wells can be damaged in a number of ways and the productivity of the well itself affected. As an example, an oil well work crew is pulling the production tubing of an oil well that is ten thousand feet deep to service the well and inspect the tubing. Tubing is usually pulled out of the well in "stands" of two or three joints. A stand is unscrewed and stood in the derrick of the rig. The tubing in the hole is suspended at the surface by the application "slips". The fluid level of the well is, for example, at seven thousand feet below the surface. The crew has all but two thousand feet of the production tubing pulled out of the hole when they fail to properly close the slips around the tool joint at the surface, and the two thousand foot length of tubing weighing about five tons, is dropped down the well. The cylindrical exterior configuration of the production tubing and its weight will cause it to hit the bottom of the well with considerable force. The fluid filling the bottom three thousand feet of the well will not have any significant effect on slowing the descent of the production tubing dropped down the well. The impact of the two thousand feet of production tubing at the bottom of the hole will result in the production tubing being corkscrewed, bent and twisted, possibly causing the casing to rupture. The twisted tubing may become jammed and stuck in the hole. The result is a very expensive fishing job, the replacement of both casing and tubing, and possibly the loss of a productive well.
For another example, take the same well during its normal production phase. The string of production tubing is 7,500 feet in length, hung at the well head and secured at the bottom with a tubing anchor, which locks onto the casing. The pump is at 7,500 feet also, at the end of the string of rods which is clamped onto the horses head of the surface pump. The tubing parts one joint from the surface, the tubing anchor fails to hold and 7,470 feet of tubing and rods, weighing about 28 tons, fall to the bottom at 10,000 feet, corkscrewing, bending and jamming inside the casing, causing the loss of the hole.